Faith is basic to most aspects of human life. When you eat, you believe that the food is good for your body. You trust those who prepare the meal. When you drink, you assume that it comes from a reasonably clean source. When you were a child, you depended on your parents or guardians. When the time came for schooling, your parents entrusted you to teachers they might or might not have known. If you married, you placed your faith in your spouse; you believed you had a future together. When your family travels, they believe the vehicle is safe. Did they watch as the vehicle was built in a factory? When the vehicle was serviced, did they see the mechanic and inspect his work? If not, your family’s feeling of safe travel depends on faith, not sight.
Faith is basic to every religion and worldview, whether that faith is in one God, or in multiple gods and spirits, or in reincarnation, or in fate, or in random chance. Scientists have faith in their intellect and assumptions, their experiments and measurements. Atheists trust their ability to discern that no God exists. Even agnostics have faith; they are sure they cannot be sure of anything.
So, faith is not unique to Christianity. What, then, is the “one faith” that unites true Christians? If the term “one faith” was written in isolation, we might be left to wonder, perhaps to invent definitions that suit our traditions. But Ephesians 4 was not written in isolation. It has a context.